The present invention relates to a method for dehydrogenating a hydrocarbon, to an apparatus useful for conducting chemical reactions, including dehydrogenation reactions, and to a method for conducting chemical reactions therein.
In the continuous catalytic dehydrogenation of a hydrocarbon such as the dehydrogenation of ethylbenzene to styrene, ethylbenzene, preheated to some temperature less than that required to thermally crack the ethylbenzene, is mixed with superheated steam and the resulting mixture immediately passed radially or axially through a bed containing a dehydrogenation catalyst. The productivity of the ethylbenzene to styrene dehydrogenation reaction is measured by the combination of conversion and selectivity. The conversion is defined as the percent of ethylbenzene which is reacted whereas the selectivity is defined as the percentage of the total reacted ethylbenzene which forms styrene.
Heretofore, various reactor systems and reaction processes have been employed to control reaction conditions, e.g., the temperature of the reaction and/or the concentration of the reactant(s) in the reaction mixture of the dehydrogenation reaction. One type of reactor system for conducting the catalytic dehydrogenation reaction comprises a massive fixed bed of catalyst wherein the heat of reaction is primarily supplied by the superheated steam mixed with the ethylbenzene feed. Due to the endothermic nature of the dehydrogenation reaction, the reaction mixture is cooled as the reaction mixture flows through the reactor and the dehydrogenation progresses. This results in a coincident reduction in the reaction rate, thereby reducing the rate at which the ethylbenzene is converted to styrene. Unfortunately, merely increasing the temperature of the initial ethylbenzene/steam mixture does not suitably eliminate this problem since the higher temperatures increase the undesirable side reactions which thereby reduce the selectivity of the dehydrogenation reaction.
To increase the conversion of the dehydrogenation reaction without significantly reducing selectivity, it has heretofore been proposed to use several catalytic reactors in series with the effluent from one reactor being preheated before entering the following reactor (see, for example, U.S. Pat. Nos. 3,660,510 and 3,755,482). In these prior art processes wherein the hydrocarbon is heated to some maximum temperature prior to contacting the catalyst bed and no additional heat is thereafter input, (except via so-called "interstage" heating), a desirable balance between conversion and selectivity cannot be achieved.
Alternatively, it has been proposed to conduct the dehydrogenation in a shell and tube reactor wherein the ethylbenzene reaction mixture flows through the tubes and the reaction mixture is heated by hot flue gases flowing on the shell side. Unfortunately, heat flux differences are exhibited across the tube bundles in the exchanger which results in different reaction rates (i.e., conversion and selectivity) in each tube, thereby preventing optimum productivity. Moreover, scale-up of the shell and tube type reactors to a production scale operation is not readily achieved.
Yet another dehydrogenation reactor is disclosed in U.S. Pat. No. 3,787,188 wherein a reactant (e.g., ethylbenzene) stream and heat maintaining fluid (e.g., superheated steam) stream are directly mixed in the presence of the dehydrogenation catalyst by flowing one of the streams axially through the catalyst bed and the other stream radially into and then axially through the bed. In the illustrated embodiment, the heat maintaining fluid is flowed upwardly through a plurality of tubes extending through the catalyst bed, for heating purposes, prior to its contact with the catalyst. The heat maintaining fluid is then passed out of the tubes into the catalyst bed through a plurality of openings in the upper portion of the tubes thereby mixing it with the reactant flowing downwardly through the catalyst bed. The conversion of the ethylbenzene is again limited in the described reactor by the maximum temperature of the steam stream.
In view of the stated deficiencies of the prior art processes for conducting dehydrogenation reactions and the apparatus used for the dehydrogenation and other reactions, it remains highly desirable to provide an economical and efficient apparatus and a process for conducting such reactions.